Method of improving the corrosion resistance of aluminum contacts on semiconductors

ABSTRACT

A method for improving the corrosion resistance of aluminum in a semiconductor device. The step includes providing a semiconductor device having aluminus containing contact pads; attaching a leadframe to the contact pads; and applying sufficient zinc chromate in solution to the contact pads for a sufficient time and at a sufficient temperature to render said aluminum substantially resistant to corrosion.

This application is a division of application Ser. No. 07/065,344, filedJun. 23, 1987 now U.S. Pat. No. 4,818,727.

BACKGROUND OF THE INVENTION

The present invention relates to semiconductor devices and moreparticularly to protective surface treatment of aluminum or aluminumalloy interconnection layers to avoid undesired corrosion. At this time,most semiconductor devices employ aluminum or aluminum alloy members asthe conductive conduit on the chip circuit. Aluminum possesses severaldesired chemical and physical properties which make it an attractivechoice for this use. These properties non-exclusively include lowresistivity, ease of deposition onto semiconductor wafer components andgood adhesion onto silicon dioxide. However, unfortunately, thinaluminum films are easily corroded in the presence of even small amountsof moisture and a small amount of an ionic species, especiallychlorides. In order to prevent the corrosion of the aluminuminterconnections on semiconductors, manufacturers usually apply acoating of glass or nitride over the top of the aluminum. This preventsthe corrosion of the aluminum except for a small area necessary toconnect the interconnect layer to the external package pins of thesemiconductor. Unfortunately, this area is especially susceptible tocorrosion. Moisture has been shown to travel along the interface betweenthe package leads and the plastic packaging material directly to theexposed aluminum at the point of connection. This corrosion is theprimary cause of failure for plastic packaged semiconductors in a moistenvironment.

Attempts by others to solve this problem require either an organicmoisture barrier, or a multilayer metal process. Many organic coatingshave been suggested as a moisture barrier including epoxies, siliconesand parylene. None have been found to be effective. All show somemoisture permeability through the material, and all allow migrationbetween the package leads and the coating at their interface.

Multilayer metal treatments have been found to be an effective techniquefor preventing the corrosion of the aluminum. These techniques apply athin barrier such as titanium or tungsten over the exposed aluminum,then coat this with a noble metal such as gold. The drawbacks to thistechnique include the expense of the complicated process of applying andpatterning these additional layers. In addition, noble metals aresusceptible to dendritic formation in moist environments. Dendriticformation may be as destructive to the microcircuit as aluminumcorrosion. The present invention provides an improved process whichrenders such exposed aluminum much less susceptible to such corrosion.In this regard, the invention relates to the use of zinc chromate topassivate the surface of the aluminum which is exposed to a corrosiveenvironment, especially at the contact pads of a semiconductor element.This zinc chromate treatment is applied to aluminum without the use oforganic binders to hold the chromate to the aluminum surface.

SUMMARY OF THE INVENTION

The invention provides a method for producing a semiconductor articlewhich comprises providing a semiconductor device having aluminumcontaining contact pads; electrically attaching a leadframe to saidcontact pads; and applying sufficient zinc chromate in solution to saidcontact pads for a sufficient time and at a sufficient temperature torender said aluminum substantially resistant to corrosion from anatmosphere containing moisture and an ionic species.

The invention also provides a method for producing an aluminumcontaining article having improved corrosion resistance which comprisesproviding an aluminum containing article and applying sufficient zincchromate in solution to said article for a sufficient time and at asufficient temperature to render said aluminum substantially resistantto corrosion from an atmosphere containing moisture and an ionicspecies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As hereinbefore mentioned, the invention concerns the use of a zincchromate solution to passivate the surface of aluminum which is exposedat the contact pads of the semiconductor device and render the samesubstantially corrosion resistant.

The zinc chromate is applied by the immersion of the semiconductormounted on its leadframe in an aqueous solution of zinc chromate at 70degrees centigrade. The semiconductor can be processed in strip formjust prior to plastic encapsulation.

During the immersion zinc deposits from the solution onto the aluminum.Additionally, the chromate acts to build the self passivation layer ofAl₂ O₃ on the aluminum.

The zinc deposited on the surface of the aluminum will later corrodeanodically to the aluminum in any corrosive atmosphere. By so doing, thezinc tends to plate out on the aluminum surface thus preventing furthercorrosion.

This is especially important for copper containing aluminum alloys.Copper containing alloys of aluminum are much more susceptible tocorrosion because copper precipitants form on the surface of thematerial and prevent the formation of the self passivation oxide layerthat allows the aluminum corrosion resistance.

The zinc chromate treatment is applied preferably in an aqueous solutioncontaining from about 0.1% to about 20% zinc chromate by weight of thesolution. In the most preferred embodiment the treatment is conducted ina saturated zinc chromate solution. Also in the preferred embodiment theapplication is conducted for from about 10 to about 60 minutes or morepreferably at from about 15 to about 20 minutes at a solutiontemperature in the range of from about 50° C. to about 100° C. Theseconditions are not critical and can be readily adjusted by those skilledin the art depending upon the effect desired and the alloy employed. Ingeneral, the leadframe and chip are subsequently encapsulated in aplastic casing. The zinc chromate applied at this stage in the processpresents few drawbacks. Experiments have been tried with immersion ofwafers prior to the interconnection of the aluminum leadframe wires. Atthis point the zinc chromate treatment was found to prevent a goodmetallurgical bond. However, if the zinc chromate is applied followingthe wire bonding operation, this is not a problem.

The zinc chromate is found to deposit only on the aluminum. No residueis found on the top surface of the passivation glass. If leakagemeasurements are taken following a zinc chromate treatment, no interleadleakage is found.

One possible drawback to the instant invention may arise if thetreatment is applied to leadframes which are to be coated with solderfollowing plastic encapsulation. This drawback is caused by thedeposition of zinc chromate on the surface of the leadframe. Unless careis taken, this residue can prevent the solder from sticking to theleadframe. However, if sufficiently active flux is used, this residuehas no effect. An alternative solution to this potential problem is toclean the leadframe in a mildly acidic wash prior to the solder coatingprocess.

The following non-limiting example serves to illustrate the invention.

EXAMPLE

A number of ceramic packaged samples of a 64k DRAM are built, but arenot lidded. Some of these samples are immersed in a saturated solutionof zinc chromate in water maintained at 70° C. for 20 minutes, while therest serve as controls. In order to perform a test of the effect of thistreatment, the cavity of each package is filled with normal tap water. Apower supply is then connected to two adjacent package pins and a fivevolt potential applied between these two pins. The parts which had notbeen exposed to the zinc chromate treatment corrode to an open circuitcondition within 30 seconds. The zinc chromated treatment samples lastmore than two hours, although some signs of corrosion are evident.

What is claimed is:
 1. A semiconductor article produced by a methodwhich comprises providing a semiconductor device having aluminumcontaining contact pads; electrically attaching a leadframe to saidcontact pads; and applying sufficient zinc chromate in solution to saidcontact pads for a sufficient time and at a sufficient temperature torender said aluminum substantially resistant to corrosion from anatmosphere containing moisture and an ionic species.
 2. The article ofclaim 1 wherein said zinc chromate in solution comprises an aqueoussolution containing from about 0.1% to about 20% zinc chromate based onthe weight of the solution component parts.
 3. The article of claim 1wherein said zinc chromate in solution comprises a saturated solution ofzinc chromate in water.
 4. The article of claim 1 wherein saidapplication is conducted wherein said solution is maintained at atemperature of from about 50° C. to bout 100°C.
 5. The article of claim1 wherein said application is conducted for from about 10 to about 60minutes.
 6. The article of claim 1 wherein said aluminum containingcontact pad is an alloy containing at least about 90% by weightaluminum.
 7. The article of claim 6 wherein said alloy contains fromabout 0.5% to about 7% by weight copper.
 8. The article of claim 6wherein said alloy contains from about 0.5% to about 1% by weightsilicon.
 9. The article of claim 1 wherein said zinc chromate insolution comprises an aqueous solution containing from about 0.1% toabout 20% zinc chromate based on the weight of the solution componentparts; and wherein said application is conducted wherein said solutionis maintained at a temperature of from bout 50° C. to about 100° C.; andwherein said application is conducted for from about 10 to bout 60minutes.
 10. A semiconductor article which comprises a semiconductordevice having aluminum containing contact pads and a leadframe havingleads which are electrically attached to a portion of said contact pads,the portion of the contact pads not underlying said leads having asurface which is the reaction product of aluminum and zinc chromate. 11.The article of claim 10 wherein said leadframe comprises aluminum andthe outer surface of said leadframe comprises the reaction product ofaluminum and zinc chromate.
 12. The article of claim 10 wherein saidaluminum containing contact pads comprise an alloy containing at leastabut 90% by weight aluminum.
 13. The article of claim 12 wherein saidalloy contains from about 0.5% to about 7% by weight copper.
 14. Thearticle of claim 12 wherein said alloy contains from about 0.5% to about1% by weight silicon.